Diabetes predominantly affects the microvascular circulation of the retina resulting in a range of structural changes that are unique to this tissue. These changes include an early persistent loss of pericytes from retinal microvessels, thickening of the basement membrane, followed by hyperproliferation of endothelial cells (ECs) and abnormal vascularization of the retina, which ultimately results in blindness. Retinal vascularization is normally restricted to the superficial and deep layers of the retina. Other ocular sites such as cornea, lens, and vitreous are normally vascular free. It has been hypothesized that a negative regulator of angiogenesis is responsible for these vascular restrictions. We have recently demonstrated that thrombospondin-1 (TSP1), a potent natural inhibitor of angiogenesis, is present at ocular avascular sites and TSPt expression is dramatically down regulated with diabetes. Our hypothesis is that TSP1 is an important modulator of retinal vascular homeostasis whose alterations under pathological conditions such as diabetes and/or ischemia results in retinal neovascularization. The studies proposed here will investigate the role of TSP1 in retinal vascular development and ischemia-induced neovascularization. We will examine the expression of TSP1 in the retina and retinal capillaries during development and determine whether its expression is altered during oxygen-induced ischemic retinopathy. We will compare development of retinal vasculature and their total area in the retina of normal, TSP1 deficient, and TSP1 over-expressing transgenic mice. We will determine whether lack or over-expression of TSP1 influences retinal vascular development and neovascularization in response to hypoxia. Identification of TSP1 as a modulator of ocular vascularization and the study of its mechanisms of action in retinal vascular cells will provide insight into the defects that contribute to retinal neovascularization. This knowledge will provide the rationale for development of new therapeutic approaches for the prevention and/or treatment of ocular diseases with a neovascular component.